With recent growing demand for flat-screen TVs, display technologies of various styles, including those making use of liquid crystal, plasma, organic electroluminescence and field emission, have been developed. For any of these displays of different display styles, a transparent electrode is an essential technology. Besides televisions, a transparent electrode is indispensable for touch panels, mobile phones, electronic paper, various solar cells and various electroluminescence control elements.
A conventional transparent electrode having been most widely used is an ITO transparent electrode produced by forming an indium-tin composite oxide (ITO) film by vacuum deposition or sputtering on a transparent substrate composed of glass, a transparent plastic film or the like. However, it has been desired to disuse indium because indium is a rare metal and its price has been soaring. There also has been desired a production technology of roll-to-roll using a flexible substrate (flexible transparent substrate), such as a resin substrate, for larger screens of displays and for improvement in productivity.
To form a transparent conductive layer, instead of the dry application methods such as vacuum deposition and sputtering, which are low in productivity, there have been developed wet application methods each for forming a transparent electrode by directly applying a dispersion containing conductive particles of a metal, metal oxide, conductive polymer or the like onto a transparent substrate and drying the same (Refer to, for example, Patent Documents 1 and 2.)
However, the wet treatment of applying and drying the dispersion raises new problems which lead to performance decrease of a transparent electrode. That is, when the dispersion containing conductive particles is dried after being applied to form a transparent conductive layer, uneven temperature and uneven drying of the coating film (applied layer) occur, which reduces thickness distribution and surface smoothness of the transparent conductive layer, thickens the applied edge part, called coffee-ring, and causes poor patterning by drying shrinkage. These lead to, when it is used as an electrode of an organic electroluminescent element (organic EL element), significant performance decrease of the element, such as increase in electrode surface resistance and occurrence of current leakage.
To deal with these problems, there are methods for increasing the drying speed and reducing the above unevenness. Examples thereof include: a method of increasing a drying temperature of conductive heat transfer drying with hot air, a hot plate or the like; and a method of carrying out radiation heat transfer drying with an infrared heater or the like. (Refer to, for example, Patent Document 3.)
However, in the case of the conductive heat transfer drying at a high temperature, when a transparent resin substrate is used as a transparent substrate, the drying treatment cannot be carried out at a temperature higher than a grass transition temperature (Tg) of the resin material because it deforms the substrate itself. Further, in a general infrared heater, when radiation energy at a wavelength of 3.0 μm, which is an adsorption region of a solvent such as water or alcohol, is increased, radiation energy at a wavelength of 5.8 μm or more, which corresponds to an absorption region of a general transparent resin substrate, is also increased, which deforms the substrate itself as with the conductive heat transfer drying at a high temperature.
As a method for effectively utilizing the radiation energy at a wavelength of 3.0 μm, there has been proposed a method using an infrared heater which absorbs infrared rays having a wavelength of 3.5 μm or more. (Refer to, for example, Patent Document 4.)
However, the main purpose of this method is to efficiently dry a solvent in an applied layer, not giving consideration to prevention of deformation of a substrate caused by the substrate itself absorbing infrared rays. In particular, infrared drying in the case where a transparent resin substrate is used is not mentioned therein.
By the way, in an organic EL element, moisture remaining in the element significantly degrades an organic layer, so that a transparent electrode and a transparent resin substrate need to be highly dried.
Usually, on a transparent resin substrate, a barrier layer is formed. Even under the drying conditions which do not clearly deform the substrate itself, the substrate may be partially infinitesimally deformed, which may separate or degrade the barrier layer. When such a transparent electrode is used in an organic EL element, the barrier function decreases and the organic layer deteriorates, which becomes a factor to cause emission unevenness and, by extension, generate dark spots.
As described above, for a transparent electrode using a transparent resin substrate for an organic EL element, high thickness uniformity, high smoothness, accurate patterning and a high degree of drying of a conductive layer as well as a high barrier property are required. It is, however, difficult for the conventional drying methods to satisfy them concurrently.